The loss of scotopic sensitivity in human patients with recessively inherited Congenital Stationary Nightblindness (CSNB) has not been thoroughly investigated in the past. This investigation will examine whether the adaptation (i.e., the regulation of sensitivity to different light levels) of residual rod signals is normal. In one experiment, the response of rod photoreceptors is studied using corneal ERG records. The rod a-wave component is isolated from the cone component by computing a "difference waveform" from the responses elicited by photopically-balanced blue and red flashes. To evaluate adaptation, rod increment threshold curves derived from the "a-wave slope" are compared for normal subjects and CSNB patients. In a second experiment, the perceptual rod signals are measured psychophysically. Rod-mediated thresholds and adaptation effects are identified by spectral and directional sensitivity measurements. Then, adaptation is evaluated by comparing the rod increment threshold curves for normal subjects and patients. Rationale: Although patients with CSNB are nightblind, they have residual rod visual function, albeit with markedly reduced sensitivity. The presence of residual rod vision in patients permits a comparison of their vision with normal subjects. Specifically, the study of adaptation attempts to evaluate whether there is a defect in the patients' visual system to regulate sensitivity at different light levels. Importance: Studies of rare visual disorders, such as CSNB, can provide a unique insight about the human visual pathways and their modes of failure. The studies can improve the understanding of how the retina adapts to different levels of light and of the measurable electrical signals generated by the retina. Advancement of the knowledge in these areas, in turn, has practical significance ot diagnostic procedures and to the development of noninvasive tests of retina function.